This invention relates to the zeolitic cation exchange of ammonium ions from aqueous solutions containing calcium cations and more particularly concerns the selective cation exchange of such solutions with certain natural or synthetic crystalline aluminosilicates. The invention is also concerned with the purification of waste waters containing ammonium cations in addition to calcium cations.
Ammonia, or the ammonium cation, has long been recognized as a serious pollutant in water. Its presence in municipal waste water and in the effluent from agricultural and industrial operations is as harmful as it is pervasive.
It has become apparent that the presence of ammonia in water has far more serious implications than merely serving as an index of recent pollution. It can be toxic to fish and aquatic life; while a maximum recommended ammonia concentration is 2.5 mg/l, as little as 0.3 to 0.4 mg/l is lethal to trout fry. It can contribute to explosive algae growths, ultimately causing eutrophic conditions in lakes. It can restrict waste water renovation and water reuse; since typical municipal waste water may contain 30 mg/l NH.sub.4, the removal of 90-95% would be required for water reuse, but to achieve this by conventional electrodialysis would be prohibitively costly. It can have detrimental effects on disinfection of water supplies; it reacts with chlorine to form chloramines which, while still bactericidal, are slower acting and less effective. Lastly, ammonia can be corrosive to certain metals and materials of construction; its effect on copper and zinc alloys is well known, and it can also be destructive to concrete made from portland cement.
Cation exchange for ammonia removal, using a variety of cation-active "zeolites", has been studied extensively but has resulted in only limited commercial utilization. The permutits (synthetic gel "zeolites" derived from sodium silicate and aluminum sulfate) and the hydrous gel-type amorphous minerals such as glauconite ("green sand") are effective but suffer from hydrolytic instability, have relatively low exchange capacity, often have other unsatisfactory regeneration characteristics, and may be difficult to form into useful shapes of acceptable physical properties. Organic "zeolites", which are sulfonated or carboxylated high polymers, are not selective for the ammonium ion, and instead prefer other cations such as calcium; In addition, their use entails excessive regenerant wastes.
Certain of the natural and synthetic crystalline aluminosilicates, which are true zeolites, have been studied for use in the selective cation exchange removal of ammonia. Fundamentally, the problem of selecting a zeolite is to obtain one having both adequate cation exchange capacity and adequate selectivity for the ammonium cation in the presence of calcium cations, which almost inevitably are present in waste water streams. Some crystalline aluminosilicates appear to possess desirable selectivity characteristics for ammonium ions in the presence of calcium ions but have relatively low exchange capacity for ammonium ions. Other commonly available crystalline zeolites appear to have an abundant initial exchange capacity for ammonium ions but poor selectivity therefor in the presence of calcium ions. Consequently in cyclic processes in which there is cumulatively an appreciable contact of these latter types of zeolites with calcium cations, there is a significant up-take of calcium ions by the zeolite and a corresponding loss of exchange capacity for ammonium ions. Moreover, the available evidence suggests that, in certain zeolites at least, the zeolitic calcium cations reduce the effectiveness of far more potential sites for ammonium ion exchange than those actually occupied by the calcium ions. Thus relatively small exchanged amounts of calcium are observed to drastically reduce the capacity of these zeolites for ammonium ion exchange even though periodic regenerations with sodium or other alkali metal ions are carried out.
It is, accordingly, an object of the invention to provide a method for the zeolitic cation exchange removal of ammonium ions from an aqueous solution containing calcium cations utilizing a zeolite possessing both high cation exchange capacity and excellent selectivity for the ammonium ion; and which zeolite has the necessary advantageous characteristics of rapid rate of exchange, ease and completeness of regeneration, stability to both the exchange solution and regenerant solutions, capability of functioning over a comparatively broad range of acidities and alkalinities, long service life, and relatively low economic cost.